Add conversation management

README.md

@@ -54,6 +54,7 @@ Go LLM Gateway (unified API)
 ✅ **Streaming support** (Server-Sent Events for all providers)
 ✅ **OAuth2/OIDC authentication** (Google, Auth0, any OIDC provider)
 ✅ **Terminal chat client** (Python with Rich UI, PEP 723)
+✅ **Conversation tracking** (previous_response_id for efficient context)
 
 ## Quick Start
 
@@ -186,8 +187,21 @@ You> /model claude
 You> /models  # List all available models
 ```
 
+The chat client automatically uses `previous_response_id` to reduce token usage by only sending new messages instead of the full conversation history.
+
 See **[CHAT_CLIENT.md](./CHAT_CLIENT.md)** for full documentation.
 
+## Conversation Management
+
+The gateway implements conversation tracking using `previous_response_id` from the Open Responses spec:
+
+- 📉 **Reduced token usage** - Only send new messages
+- ⚡ **Smaller requests** - Less bandwidth
+- 🧠 **Server-side context** - Gateway maintains history
+- ⏰ **Auto-expire** - Conversations expire after 1 hour
+
+See **[CONVERSATIONS.md](./CONVERSATIONS.md)** for details.
+
 ## Authentication
 
 The gateway supports OAuth2/OIDC authentication. See **[AUTH.md](./AUTH.md)** for setup instructions.
@@ -216,6 +230,8 @@ curl -X POST http://localhost:8080/v1/responses \
 
 - ✅ ~~Implement streaming responses~~
 - ✅ ~~Add OAuth2/OIDC authentication~~
+- ✅ ~~Implement conversation tracking with previous_response_id~~
 - ⬜ Add structured logging, tracing, and request-level metrics
 - ⬜ Support tool/function calling
+- ⬜ Persistent conversation storage (Redis/database)
 - ⬜ Expand configuration to support routing policies (cost, latency, failover)

cmd/gateway/main.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/yourusername/go-llm-gateway/internal/auth"
 	"github.com/yourusername/go-llm-gateway/internal/config"
+	"github.com/yourusername/go-llm-gateway/internal/conversation"
 	"github.com/yourusername/go-llm-gateway/internal/providers"
 	"github.com/yourusername/go-llm-gateway/internal/server"
 )
@@ -47,7 +48,11 @@ func main() {
 		logger.Printf("Authentication disabled - WARNING: API is publicly accessible")
 	}
 
-	gatewayServer := server.New(registry, logger)
+	// Initialize conversation store (1 hour TTL)
+	convStore := conversation.NewStore(1 * time.Hour)
+	logger.Printf("Conversation store initialized (TTL: 1h)")
+
+	gatewayServer := server.New(registry, convStore, logger)
 	mux := http.NewServeMux()
 	gatewayServer.RegisterRoutes(mux)
 

internal/api/types.go

@@ -13,6 +13,7 @@ type ResponseRequest struct {
 	Metadata           map[string]string `json:"metadata,omitempty"`
 	Input              []Message         `json:"input"`
 	Stream             bool              `json:"stream,omitempty"`
+	PreviousResponseID string            `json:"previous_response_id,omitempty"`
 }
 
 // Message captures user, assistant, or system roles.

internal/conversation/conversation.go

@@ -0,0 +1,112 @@
+package conversation
+
+import (
+	"sync"
+	"time"
+
+	"github.com/yourusername/go-llm-gateway/internal/api"
+)
+
+// Store manages conversation history with automatic expiration.
+type Store struct {
+	conversations map[string]*Conversation
+	mu            sync.RWMutex
+	ttl           time.Duration
+}
+
+// Conversation holds the message history for a single conversation thread.
+type Conversation struct {
+	ID        string
+	Messages  []api.Message
+	Model     string
+	CreatedAt time.Time
+	UpdatedAt time.Time
+}
+
+// NewStore creates a conversation store with the given TTL.
+func NewStore(ttl time.Duration) *Store {
+	s := &Store{
+		conversations: make(map[string]*Conversation),
+		ttl:           ttl,
+	}
+	
+	// Start cleanup goroutine
+	go s.cleanup()
+	
+	return s
+}
+
+// Get retrieves a conversation by ID.
+func (s *Store) Get(id string) (*Conversation, bool) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	
+	conv, ok := s.conversations[id]
+	return conv, ok
+}
+
+// Create creates a new conversation with the given messages.
+func (s *Store) Create(id string, model string, messages []api.Message) *Conversation {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	
+	now := time.Now()
+	conv := &Conversation{
+		ID:        id,
+		Messages:  messages,
+		Model:     model,
+		CreatedAt: now,
+		UpdatedAt: now,
+	}
+	
+	s.conversations[id] = conv
+	return conv
+}
+
+// Append adds new messages to an existing conversation.
+func (s *Store) Append(id string, messages ...api.Message) (*Conversation, bool) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	
+	conv, ok := s.conversations[id]
+	if !ok {
+		return nil, false
+	}
+	
+	conv.Messages = append(conv.Messages, messages...)
+	conv.UpdatedAt = time.Now()
+	
+	return conv, true
+}
+
+// Delete removes a conversation from the store.
+func (s *Store) Delete(id string) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	
+	delete(s.conversations, id)
+}
+
+// cleanup periodically removes expired conversations.
+func (s *Store) cleanup() {
+	ticker := time.NewTicker(1 * time.Minute)
+	defer ticker.Stop()
+	
+	for range ticker.C {
+		s.mu.Lock()
+		now := time.Now()
+		for id, conv := range s.conversations {
+			if now.Sub(conv.UpdatedAt) > s.ttl {
+				delete(s.conversations, id)
+			}
+		}
+		s.mu.Unlock()
+	}
+}
+
+// Size returns the number of active conversations.
+func (s *Store) Size() int {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return len(s.conversations)
+}

internal/providers/anthropic/anthropic.go

@@ -53,7 +53,7 @@ func (p *Provider) Generate(ctx context.Context, req *api.ResponseRequest) (*api
 	for _, msg := range req.Input {
 		var content string
 		for _, block := range msg.Content {
-			if block.Type == "input_text" {
+			if block.Type == "input_text" || block.Type == "output_text" {
 				content += block.Text
 			}
 		}
@@ -147,7 +147,7 @@ func (p *Provider) GenerateStream(ctx context.Context, req *api.ResponseRequest)
 		for _, msg := range req.Input {
 			var content string
 			for _, block := range msg.Content {
-				if block.Type == "input_text" {
+				if block.Type == "input_text" || block.Type == "output_text" {
 					content += block.Text
 				}
 				}

internal/providers/google/google.go

@@ -54,11 +54,21 @@ func (p *Provider) Generate(ctx context.Context, req *api.ResponseRequest) (*api
 
 	// Convert Open Responses messages to Gemini format
 	var contents []*genai.Content
+	var systemText string
 	
 	for _, msg := range req.Input {
+		if msg.Role == "system" {
+			for _, block := range msg.Content {
+				if block.Type == "input_text" || block.Type == "output_text" {
+					systemText += block.Text
+				}
+			}
+			continue
+		}
+
 		var parts []*genai.Part
 		for _, block := range msg.Content {
-			if block.Type == "input_text" {
+			if block.Type == "input_text" || block.Type == "output_text" {
 				parts = append(parts, genai.NewPartFromText(block.Text))
 			}
 		}
@@ -74,8 +84,18 @@ func (p *Provider) Generate(ctx context.Context, req *api.ResponseRequest) (*api
 		})
 	}
 
+	// Build config with system instruction if present
+	var config *genai.GenerateContentConfig
+	if systemText != "" {
+		config = &genai.GenerateContentConfig{
+			SystemInstruction: &genai.Content{
+				Parts: []*genai.Part{genai.NewPartFromText(systemText)},
+			},
+		}
+	}
+
 	// Generate content
-	resp, err := p.client.Models.GenerateContent(ctx, model, contents, nil)
+	resp, err := p.client.Models.GenerateContent(ctx, model, contents, config)
 	if err != nil {
 		return nil, fmt.Errorf("google api error: %w", err)
 	}
@@ -143,11 +163,21 @@ func (p *Provider) GenerateStream(ctx context.Context, req *api.ResponseRequest)
 
 		// Convert messages
 		var contents []*genai.Content
+		var systemText string
 		
 		for _, msg := range req.Input {
+			if msg.Role == "system" {
+				for _, block := range msg.Content {
+					if block.Type == "input_text" || block.Type == "output_text" {
+						systemText += block.Text
+					}
+				}
+				continue
+			}
+
 			var parts []*genai.Part
 			for _, block := range msg.Content {
-				if block.Type == "input_text" {
+				if block.Type == "input_text" || block.Type == "output_text" {
 					parts = append(parts, genai.NewPartFromText(block.Text))
 				}
 			}
@@ -163,8 +193,18 @@ func (p *Provider) GenerateStream(ctx context.Context, req *api.ResponseRequest)
 			})
 		}
 
+		// Build config with system instruction if present
+		var config *genai.GenerateContentConfig
+		if systemText != "" {
+			config = &genai.GenerateContentConfig{
+				SystemInstruction: &genai.Content{
+					Parts: []*genai.Part{genai.NewPartFromText(systemText)},
+				},
+			}
+		}
+
 		// Create stream
-		stream := p.client.Models.GenerateContentStream(ctx, model, contents, nil)
+		stream := p.client.Models.GenerateContentStream(ctx, model, contents, config)
 
 		// Process stream
 		for resp, err := range stream {

internal/providers/openai/openai.go

@@ -52,7 +52,7 @@ func (p *Provider) Generate(ctx context.Context, req *api.ResponseRequest) (*api
 	for _, msg := range req.Input {
 		var content string
 		for _, block := range msg.Content {
-			if block.Type == "input_text" {
+			if block.Type == "input_text" || block.Type == "output_text" {
 				content += block.Text
 			}
 		}
@@ -127,7 +127,7 @@ func (p *Provider) GenerateStream(ctx context.Context, req *api.ResponseRequest)
 		for _, msg := range req.Input {
 			var content string
 			for _, block := range msg.Content {
-				if block.Type == "input_text" {
+				if block.Type == "input_text" || block.Type == "output_text" {
 					content += block.Text
 				}
 				}

internal/server/server.go

@@ -7,18 +7,24 @@ import (
 	"net/http"
 
 	"github.com/yourusername/go-llm-gateway/internal/api"
+	"github.com/yourusername/go-llm-gateway/internal/conversation"
 	"github.com/yourusername/go-llm-gateway/internal/providers"
 )
 
 // GatewayServer hosts the Open Responses API for the gateway.
 type GatewayServer struct {
 	registry *providers.Registry
+	convs    *conversation.Store
 	logger   *log.Logger
 }
 
 // New creates a GatewayServer bound to the provider registry.
-func New(registry *providers.Registry, logger *log.Logger) *GatewayServer {
-	return &GatewayServer{registry: registry, logger: logger}
+func New(registry *providers.Registry, convs *conversation.Store, logger *log.Logger) *GatewayServer {
+	return &GatewayServer{
+		registry: registry,
+		convs:    convs,
+		logger:   logger,
+	}
 }
 
 // RegisterRoutes wires the HTTP handlers onto the provided mux.
@@ -43,6 +49,17 @@ func (s *GatewayServer) handleResponses(w http.ResponseWriter, r *http.Request)
 		return
 	}
 
+	// Build full message history
+	messages := s.buildMessageHistory(&req)
+	if messages == nil {
+		http.Error(w, "conversation not found", http.StatusNotFound)
+		return
+	}
+
+	// Update request with full history for provider
+	fullReq := req
+	fullReq.Input = messages
+
 	provider, err := s.resolveProvider(&req)
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusBadGateway)
@@ -51,26 +68,58 @@ func (s *GatewayServer) handleResponses(w http.ResponseWriter, r *http.Request)
 
 	// Handle streaming vs non-streaming
 	if req.Stream {
-		s.handleStreamingResponse(w, r, provider, &req)
+		s.handleStreamingResponse(w, r, provider, &fullReq, &req)
 	} else {
-		s.handleSyncResponse(w, r, provider, &req)
+		s.handleSyncResponse(w, r, provider, &fullReq, &req)
 	}
 }
 
-func (s *GatewayServer) handleSyncResponse(w http.ResponseWriter, r *http.Request, provider providers.Provider, req *api.ResponseRequest) {
-	resp, err := provider.Generate(r.Context(), req)
+func (s *GatewayServer) buildMessageHistory(req *api.ResponseRequest) []api.Message {
+	// If no previous_response_id, use input as-is
+	if req.PreviousResponseID == "" {
+		return req.Input
+	}
+
+	// Load previous conversation
+	conv, ok := s.convs.Get(req.PreviousResponseID)
+	if !ok {
+		return nil
+	}
+
+	// Append new input to conversation history
+	messages := make([]api.Message, len(conv.Messages))
+	copy(messages, conv.Messages)
+	messages = append(messages, req.Input...)
+
+	return messages
+}
+
+func (s *GatewayServer) handleSyncResponse(w http.ResponseWriter, r *http.Request, provider providers.Provider, fullReq *api.ResponseRequest, origReq *api.ResponseRequest) {
+	resp, err := provider.Generate(r.Context(), fullReq)
 	if err != nil {
 		s.logger.Printf("provider %s error: %v", provider.Name(), err)
 		http.Error(w, "provider error", http.StatusBadGateway)
 		return
 	}
 
+	// Store conversation - use previous_response_id if continuing, otherwise use new ID
+	conversationID := origReq.PreviousResponseID
+	if conversationID == "" {
+		conversationID = resp.ID
+	}
+	
+	messages := append(fullReq.Input, resp.Output...)
+	s.convs.Create(conversationID, resp.Model, messages)
+	
+	// Return the conversation ID (not the provider's response ID)
+	resp.ID = conversationID
+
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(http.StatusOK)
 	_ = json.NewEncoder(w).Encode(resp)
 }
 
-func (s *GatewayServer) handleStreamingResponse(w http.ResponseWriter, r *http.Request, provider providers.Provider, req *api.ResponseRequest) {
+func (s *GatewayServer) handleStreamingResponse(w http.ResponseWriter, r *http.Request, provider providers.Provider, fullReq *api.ResponseRequest, origReq *api.ResponseRequest) {
 	// Set headers for SSE
 	w.Header().Set("Content-Type", "text/event-stream")
 	w.Header().Set("Cache-Control", "no-cache")
@@ -83,7 +132,10 @@ func (s *GatewayServer) handleStreamingResponse(w http.ResponseWriter, r *http.R
 		return
 	}
 
-	chunkChan, errChan := provider.GenerateStream(r.Context(), req)
+	chunkChan, errChan := provider.GenerateStream(r.Context(), fullReq)
+	
+	var responseID string
+	var fullText string
 
 	for {
 		select {
@@ -92,6 +144,25 @@ func (s *GatewayServer) handleStreamingResponse(w http.ResponseWriter, r *http.R
 				return
 			}
 			
+			// Capture response ID
+			if chunk.ID != "" && responseID == "" {
+				responseID = chunk.ID
+			}
+			
+			// Override chunk ID with conversation ID
+			if origReq.PreviousResponseID != "" {
+				chunk.ID = origReq.PreviousResponseID
+			} else if responseID != "" {
+				chunk.ID = responseID
+			}
+			
+			// Accumulate text from deltas
+			if chunk.Delta != nil && len(chunk.Delta.Content) > 0 {
+				for _, block := range chunk.Delta.Content {
+					fullText += block.Text
+				}
+			}
+			
 			data, err := json.Marshal(chunk)
 			if err != nil {
 				s.logger.Printf("failed to marshal chunk: %v", err)
@@ -102,6 +173,8 @@ func (s *GatewayServer) handleStreamingResponse(w http.ResponseWriter, r *http.R
 			flusher.Flush()
 			
 			if chunk.Done {
+				// Store conversation with a single consolidated assistant message
+				s.storeStreamConversation(fullReq, origReq, responseID, fullText)
 				return
 			}
 			
@@ -112,6 +185,8 @@ func (s *GatewayServer) handleStreamingResponse(w http.ResponseWriter, r *http.R
 				fmt.Fprintf(w, "data: %s\n\n", errData)
 				flusher.Flush()
 			}
+			// Store whatever we accumulated before the error
+			s.storeStreamConversation(fullReq, origReq, responseID, fullText)
 			return
 			
 		case <-r.Context().Done():
@@ -121,6 +196,27 @@ func (s *GatewayServer) handleStreamingResponse(w http.ResponseWriter, r *http.R
 	}
 }
 
+func (s *GatewayServer) storeStreamConversation(fullReq *api.ResponseRequest, origReq *api.ResponseRequest, responseID string, fullText string) {
+	if responseID == "" || fullText == "" {
+		return
+	}
+
+	assistantMsg := api.Message{
+		Role: "assistant",
+		Content: []api.ContentBlock{
+			{Type: "output_text", Text: fullText},
+		},
+	}
+	messages := append(fullReq.Input, assistantMsg)
+
+	conversationID := origReq.PreviousResponseID
+	if conversationID == "" {
+		conversationID = responseID
+	}
+
+	s.convs.Create(conversationID, fullReq.Model, messages)
+}
+
 func (s *GatewayServer) resolveProvider(req *api.ResponseRequest) (providers.Provider, error) {
 	if req.Provider != "" {
 		if provider, ok := s.registry.Get(req.Provider); ok {